50mi really isnt that long of a ride in terms of distance. Water is the most important carry-on. Unless you are diabetic, snacks rate a #2 on the importance list. You dont need fancy gels or pasty tasting energy bars. Granola snack bars are perfect as are fruit snacks of all types. No need to make this more expensive than necessary and do remember that life is too short and cycling too much fun for snacks that taste like crap.
You are doing fine with the mileage.....big congrats on that. Likewise, you will figure out what you will need for the distances and what tastes good. Dont wait for the big ride day to try something new for snacks.
As a point of interest....it was the 50mi mark that defined "long distance" to this roadie. Something happens to your body's demands at and above the half-century mark. Listen to your body and keep riding.